This invention relates to a method of cast molding toric contact lenses.
Contact lenses having a toric optical zone (commonly referred to as "toric contact lenses") are used to correct refractive abnormalities of the eye associated with astigmatism. The toric optical zone provides cylindrical correction to compensate for the astigmatism. Since astigmatism requiring vision correction is usually associated with other refractive abnormalities, such as myopia (nearsightedness) or hypermetropia (farsightedness), toric contact lenses are generally prescribed also with a spherical correction to correct myopic astigmatism or hypermetropic astigmatism. Currently, both back toric lenses (having the toric surface formed in the posterior lens surface) and front toric lenses (having the toric surface formed in the anterior lens surface) are available.
Whereas spherical contact lenses may freely rotate on the eye, toric contact lenses have a ballast to inhibit rotation of the lens on the eye so that the cylindrical axis of the toric zone remains generally aligned with the axis of the astigmatism. For example, a section of the lens periphery may be thicker (or thinner) than another section to provide the ballast. Toric contact lenses are manufactured with a selected relationship (or offset) between the cylindrical axis of the toric optical zone and the orientation of the ballast. This relationship is expressed as the number of degrees (rotational angle) that the cylindrical axis is offset from the orientation axis of the ballast.
Toric contact lenses, similar to spherical contact lenses, are usually offered in several different base curves, a lens parameter related to fitting characteristics of the lens, and several different spherical powers. Toric contact lenses, however, not only include the toric optical zone and ballast, but the lenses are offered with a range of cylindrical axis orientations in order to accommodate patients with different astigmatic conditions; for example, for a given base curve and spherical power, the cylindrical axis may be offered in 5 or 10 degree increments ranging from 0.degree. to 180.degree.. Toric contact lenses are often referred to as a "specialty lens" product line; many individual prescriptions are offered, and the number of individual lenses manufactured, for an individual prescription, is very small when compared to spherical contact lenses.
Various methods are used for manufacturing contact lenses. The majority of toric contact lenses are produced by lathing a button (or lens blank), or a semi-finished button (containing one finished surface). See, for example, U.S. Pat. No. 4,680,998 (Council, Jr.).
Another method known, in general, for manufacturing contact lenses is cast molding. Cast molding of contact lenses involves depositing a curable mixture of polymerizable monomers in a mold cavity formed by two mold sections, curing the monomer mixture, and disassembling the mold assembly and removing the lens. Other processing steps, for example, hydration in the case of hydrogel lenses, may also be employed. One mold section forms the anterior lens surface (anterior mold section), and the other mold section forms the posterior lens surface (posterior mold section).
Prior to the cast molding of the contact lens, each of the mold sections is formed by injection molding a resin in the cavity of an injection molding apparatus. Mounted in the injection molding apparatus are tools for forming the optical surface on the mold sections. Whereas the mold sections are typically used only once for casting a lens, the injection molding tools are used to make hundreds of molds. The tools must be manufactured to extremely high specifications so that no roughness or surface defects are transferred to the mold section being made therefrom, as any such defects on the mold surface would be transferred to the molded contact lens. The tools are typically made from brass, stainless steel, nickel or some combination thereof.
Several known cast molding methods have the potential to mold a finished contact lens. As an example, U.S. Pat. No. 5,271,875 (Appleton et al.) discloses such a cast molding method. Since these methods avoid time-consuming and labor-intensive operations such as lathing, the methods have been found to offer the potential to reduce production time and cost for the manufacture of spherical contact lenses.
However, various problems have been encountered in employing cast molding technology for manufacturing toric contact lenses.
If one wished to manufacture a toric contact lens product line by cast molding, the number of unique tools that must be designed, manufactured and stocked is significantly high. For example, assuming that, for a given base curve and optical power, toric contact lenses are offered with cylindrical axes offset from the ballast in 10-degree increments ranging from 0 to 180 degrees, the number of sets of tools for producing the sets of anterior and posterior mold sections would be on the order of 18 times greater than needed for spherical contact lenses of the same base curve and power. These considerations are further complicated by the fact that for toric contact lenses, the demand for an individual prescription is much lower than for spherical lenses.
U.S. Pat. No. 5,252,056 (Horner et al.) discloses a contact lens cast molding method employing two mold halves. When the two mold halves are joined together, vertical rib-like fixing elements on one mold half and a cylindrical projecting extension on the other mold half adhere to each other solely by frictional contact. The disclosure mentions that the two mold halves can be joined together only in a very specific orientation relative to each other, whereby toric contact lenses can be manufactured. While this approach may permit alignment of the two mold sections at a specific orientation, the number of sets of tools required to cast mold a toric lens product line would remain significantly high.